1. Field of the Invention
The present invention relates to an illumination system for reflecting light from a light source (or a secondary light source) by a reflecting optical element such as a spherical mirror, a parabolic mirror or the like and for directing reflected light with parallel rays toward a surface to be illuminated.
2. Description of the Prior Art
Exposure apparatus is used for printing a prescribed pattern on a printed board, a liquid crystal substrate or the like. It is regarded as preferable that an illumination system for use within the exposure apparatus be capable of directing parallel rays onto a surface of the printed board, for example. To satisfy this requirement, the illumination system generally utilizes a reflection type collimator.
FIG. 1 schematically illustrates the structure of an illumination system including a reflection type collimator, which provides a background for the understanding of the present invention. Referring to FIG. 1, the illumination system includes a spherical mirror 1 and a secondary light source 2. The secondary light source 2 is located at a point close to a focal point of the spherical mirror 1 and deviated from a principal axis 1b thereof. The principal axis 1b is defined as a symmetry axis through a center of symmetry for the spherical mirror 1. Illuminating light from the secondary light source 2 is reflected by a reflecting surface 1a of the spherical mirror 1, and then guided toward an illuminated surface 3. As seen in FIG. 1, the spherical mirror 1 is so arranged that its principal axis 1b is inclined at a prescribed angle .theta. to an optical axis 4 of the illuminating system. In order to simplify the following description, the angle .theta. is hereinafter referred to an off-axis angle.
The size of the illumination system shown in FIG. 1 is increased proportionally to a distance L between the spherical mirror 1 and the illuminated surface 3. Consequently, an exposure apparatus, to which the illumination system is applied, attains a great size as the distance L is increased. In general, however, such an exposure apparatus be of small size. Accordingly, the distance L between the spherical mirror 1 and the illuminated surface 3 is generally set at a relatively small value as compared with a focal length f of the spherical mirror 1.
As is well known, the illuminated surface 3 must be illuminated as uniformly as possible. It has been considered difficult to illuminate the surface 3 with uniform illuminance by merely appropriately setting designed values, such as the distance L, in the illumination system, because the secondary light source 2 is located on the point deviated from the principal axis 1b of the spherical mirror 1.
The foregoing problem may be overcome in the following manner: The illumination system is designed so that illuminance distribution on the illuminated surface 3 is symmetrical with respect to the optical axis of the illuminating system. Furthermore, an appropriate element is added to said illumination system. The element maybe a fly-eye lens, with a characteristic of correcting illuminance distribution on the illuminated surface 3, which is used as the secondary light source 2. Thus, the illuminated surface 3 can be illuminated with uniform illuminance. In such an approach, it is desired that the illuminance distribution on the illuminated surface 3 is substantially symmetrical with respect to the optical axis of the illuminating system.
The conventional illumination system is merely designed so that the distance L is relatively shorter than the focal length f of the spherical mirror 1 in order to simply reduce the size of the apparatus; no consideration is made as to the resultant illuminance distribution on the illuminated surface 3. If, illuminance distribution on the illuminated surface 3 is asymmetrical with respect to the optical axis of the illuminating system, as shown in FIG. 2 for example, such asymmetry causes a significant problem as an effective illuminated region of the illuminated surface 3 is increased, but substantially no problem takes place if the effective illuminated region is small. Referring to the effective illuminated region having dimensions of 0.4 f by 0.4 f, for example, difference in illuminance between both ends (points separated from the optical axis at distances of -0.2 f and 0.2 f) is .DELTA.I.sub.1 as shown in FIG. 2. On the other hand, such difference is .DELTA.I.sub.2 (&gt;.DELTA.I.sub.1) if the dimensions of the effective illuminated region is set at 0.8 f by 0.8 f. As will be understood from FIG. 2, the difference in illuminance between the ends of the effective illuminated region is increased as the effective illuminated region is increased in size when illuminance distribution is asymmetrical. Particularly with increase in size of the printed board etc. to be subjected to pattern printing, the region to be illuminated with parallel rays is widened to increasingly raise the problem of asymmetrical illuminance distribution in the effective illuminated region.